metering 18 per cent gray cards

Discussion in 'Large Format' started by roger_hicks|1, Nov 16, 2003.

  1. Would anyone care to explain to me, in their own words -- ie without
    telling me to read Ansel Adamas, Minor White, Zakia, Lorenz et al --
    how metering an 18 per cent gray card is supposed to give an optimum
    exposure in all situations in black and white photography?
     
  2. simply - it doesn't.
     
  3. Well I'm no expert but the principle is something ,like this. 18% grey roughly represents half way between black and white. Assuming a scene you were photographing is a mixture of equal blacks,greys and whites, then reading a reflective light meter reading off a grey card should in theory give proper exposure for such a scene. This is because modern camera metering systems are I understand calibrated to calculate correct exposure of an 18% grey surface. The meter assumes that every scene is a mid grey .

    Therefore lets assume we are taking a shot of a piece of card, the card has 3 bands of equal size, one white, one 18% grey and one black. If we take a reading off the central 18% grey section , then all 3 bands should be correctly exposed. Taking a reading from the white section would render the white section grey, the grey section a much darker grey and the black section, well black but underexposed. Taking a reading from the black section would render the black are now grey, the 18% grey section almost white, and the white section an overexposed white.

    I think this is the theory, I fell sure I will be corrected if I am wrong.


    Max
     
  4. Many years ago, before color photography. thousands of photographic scenes were
    analyzed as to their mixture of black and white and it was determined that the
    average was equivalent to 18% grey. Therefore if you hold an 18% grey card in the
    same light as the main subject and take a meter reading from the card it will
    represent the correct exposure for the average scene.

    If you use an incident meter at the location of the grey card, the meter is calibrated
    as though it was an average subject and it measures the light that will be needed to
    give the correct exposure for an average subject.
     
  5. Riger -

    (Are you by any chance related to the Roger Hicks who writes photography articles in the UL?)

    Light meters respond to the intensity of light. But for them to be useful, they have to be calibrated to a standard. That is, when the meter measures the intensity of the light, and translates that measurement into a recommended exposure setting, there needs to be some arbitrary reference. That reference is a surface that reflects 18% of the light that falls on it. So when you measure the light on an object with a light meter, and expose exactly as the light meter tells you, the resulting image on film appears as though the object is reflecting 18% of the light falling on it.

    Now, the problem is that all objects don't reflect 18% of the light - some reflect more (and should appear on the film as brighter than 18% reflectance), while others reflect less (and should appear darker).

    So the theory behind using an 18% reflectance gray card is that if you measure the light relfecting from something that is known to have 1% reflectance, and expose accordingly, it will show up on the film with the brightness appropriate to 18% reflectance. And objects that reflect more or less light will appear with relative brightnesses appropriate to their reflectance.
     
  6. Come on, Roger, you know the answer. Are you preparing to write an article about common confusions?

    Metering off a grey card is a substitute for metering incident light. That's all. Don't have an incident meter, do have a camera with on-board meter or a meter that does only reflected? A card will cost less.

    Cheers,

    Dan
     
  7. Although the previous responses actually cover the subject, I'll take a slightly different route to the same destination.

    Although there is some disagreement about what reflective meters are calibrated to (18% vs. 13%, or something like that), the 18% gray card represents a reasonable approximation of a "mid tone" value, arguably a Zone V in Zone System terms. Thus, metering off a surface of known reflectivity that closely matches the calibration of the meter gives a reasonable "average" exposure - essentially the equivalent of a proper measurement with an incident meter. That approach, of course, exposes for a range of values centered on the "average" of the 18% gray card, and lets the shadows and highlights fall where they will in relation to that "average" value. So, depending on the range of actual values in the scene, that exposure setting may or may not be "optimal". One might actually want to either lean toward preserving highlight detail, or shadow detail, for example. Or, use the Zone System to try to squeeze the maximum amount of pictorial data onto the negative.

    If the gray-card approach gave universally optimal exposures, of course, the Zone System never would have been invented. But, it's a reasonable starting point, and is probably more reliable than guessing which leaf or flower will translate to a middle gray.
     
  8. 18 percent gray is the ideal mid-point of an ideal 5 stop range. (Math available on request.) Most light meters are calibrated to this 18% reference (or something close) for the purpose of indicating proper exposure for this 5-stop range. Even if conditions differ from ideal, using this refrence point will get most exposures in the ballpark (color too).

    If being in the ballpark is the definition for optimum, then the arrangement is probably fair. If optimum means getting the best exposure for conditions, then allowances are likely to be needed. Conditions may include not only the lighting, but light meter response, film behavior, development, printing, and personal taste.
     
  9. jbq

    jbq

    Metering 18 percent in B&W will give you the otimum exposure if you photograph an evenly lit subject with a brightness range going all the way from black to white, if you develop your film normally, print it on normally exposed and normally developped grade 3 paper.

    The beauty of B&W is that you can control many of the parameters, and that you can adapt the to your subject.

    In large format B&W it would be sad to not get at least some understanding of the zone system. The hard (tedious) part of the zone system is to figure out the development times. If you have a lab that knows what times to apply for a specific kind of film, go ahead, use that film, apply the zone system, give them your film, and enjoy!
     
  10. Hello Roger,

    After writing the book "Perfect Exposure" I would have thought that you would be providing the answers ... and not asking the questions.

    As mentioned by an earlier post... the gray card does not give an optimum exposure in all situations.

    Kind Regards,
     
  11. I have some doubt that this is THE Roger Hicks - the real Roger Hicks writes in English, not American English, and I doubt that he would be so unsubtle in pushing his own books. Although he is right, of course.
     
  12. Roger

    Given your recent opinionated articles in "Amateur Photographer", I suspect that your message is intended to elicit material to further your argument that large format photographers are a foolish bunch.

    I would simply ask that, should this be the case, you have the decency to make a contribution to photo.net for the resource you are taking advantage of.

    I am well aware that you are a photographer of some considerable standing. Nevertheless, if you would like to have a sensible discussion about the single most significant advantage of view cameras - the ability to place the plane of sharp focus - I would welcome you to visit me in Tintern, UK, to conduct it.

    It's my suspicion that should you ever feel that you might be open to learning something new about photography, and I get a chance to try and explain the Hinge rule and how to apply it practically, then you might not be quite so dismissive about large format photographers in your articles in the photographic press in the future.

    Some of the regular forum participants may raise an eyebrow at my judgemental tone - I have been promoting friendliness in this forum for a while now. That friendliness will accomodate you as soon as I know that you are asking a genuine question and that participants taking the trouble to answer are aware of your agenda.
     
  13. Judging from the question, I guess "The Perfect Exposure" is not so perfect....
     
  14. Rob Barker, haven't you learned the difference between an author and an authority?
     
  15. I have the utmost respect for Roger's talent and achievements. The fact is that his trolling articles in AP recently lead me to suspect his motivation for posting in here - he may like to clarify it. It could be that I have the wrong end of the stick and the intention of starting the thread was to provoke a good honest debate. I wouldn't wish to see participants here belittled in future articles though.
     
  16. If this is THE Roger Hicks, I suggest you read books written by THE authorities! I think you'll find that Adams suggests employing a method called "The Zone System"> You may have heard of it? If you have any difficulty with exposure and large format photography then I heartily suggest taking up the offer from Rob and come along to an F32 workshop........we'll show you how its done.
     
  17. Now I am uncertain if this is THE Roger Hicks. If it is though, I must say I rather enjoyed the book "Perfect Exposure" and learnt a great deal from reading the material.

    I am unaware of the article a previous poster is making reference to. Could somebody enlighten me?

    If this is the real Roger Hicks then I would like to make it clear that in no manner was I intending to incite any negative responses. On the contrary... I am quite a fan of your publications.

    Kind Regards,
     
  18. It's my suspicion that should you ever feel that you might be open to learning something new about photography, and I get a chance to try and explain the Hinge rule and how to apply it practically, then you might not be quite so dismissive about large format photographers in your articles in the photographic press in the future.
    Rob, if he is "The" Roger Hicks.....he has been a large format photographer for about 30 years.
    In fact it was his article about 5x7 cameras....that got me started on the format.
    He is also the author of such articles as "Why 4x5 is lighter than 35mm", and wrote a very discriptive article about one of his favorite cameras the Linhof Technika.
    Roger...if you are "Roger" than let me tell you it was your articles that keep me buying Shutterbug......
     
  19. Let me make it clear that I am a great admirer of much of Roger Hick's writing (I don't agree with it all, as I don't believe in gurus - as someone once said), I just don't believe he would troll like this.

    However, it's true that the 18% grey value is pretty pointless, as it's based on a wrong assumption.
     
  20. If you follow some of Roger's previous postings on Photonet you will find that he has in the past offered advice in the form of 'read my book ....' which indeed turn out to be Hicks/Schultz published by David and Charles.

    I think that establishes that we have the real Roger, unless someone is deliberately 'passing-off'.

    My esteem for RH was seriously dented by his AP musings on Large Format, but everyone is entitled to their view.

    So the question remains, why the question?
     
  21. The question seems to me to be evocative of either one of two mindsets, and this may be a distinction without a difference, but one.......'without resorting to dogma(from the point of view of the question, not me), can anybody explain this?.......and/or.........he/whoever he is doesn't believe it can be explained by any of the folks mentioned or anybody here.
     
  22. The question assumes a faulty premise is a way in that since the combination of a meter(calibrated or not) and a human being/Photographer using that meter to measure something is going to be an interpretation of something where 'correct' has nothing to do with it, the need to be precise as possible in getting what you want notwithstanding.

    Coming up with the correct exposure(whatever that means) doesn't always mean it will 'right', take situations of being outside where the sun is your primary light source(and you're not using a reflector), but is not behind you but creating a 'sidelight' or 'silhouette', you may have to 'bias' in one direction or another despite what the meter says, that's a choice away from what the meter says, and further you make another choice by determining how much, despite calibrated meters, and any/every book ever written(which doesn't mean they shouldn't be read), the are of metering for an exposure is bottom line guestimation and interpretation to differing extents dependant upon the scene.

    I'm no expert on the zone system, I barely understand the bare rudiments, but I've audited several threads describing individuals suggesting a switch from one zone to another for such and such a reason, in portraiture the what the meter says may result in a flat dull shot, by the same token the question is meaningless and makes no difference if by using your tools you've gotten a well technically crafted shot, your head will always be the determining factor after you've read whatever you've read and regardless of whether you're using a meter and an %18 grey card or not.
     
  23. With due respect, this last response has given me a headache. I hope I can sleep tonight :)
     
  24. The point of the question is simple, I believe, and well worth asking - 18% grey does not relate to an average subject, for one thing - I believe 12% is closer. Secondly, how often is the subject you're attempting to meter an even mix of tones? If it's not, what sort of exposure will you get?
     
  25. Having done some more research I see where Roger's question has come from and it's from this thread :-

    http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=006W9S

    The opportunity to debate some of these issues helps everybody's growth and understanding. The chance to debate that with someone of Roger's pre-eminence is a total privilege of the internet and these forums, let's take the responsibility that that freedom allows and behave in a tolerant and 'professional' manner.

    I respect Roger's willingness to participate here and the previous thread referred to above hopefully puts this thread now in context.
     
  26. Roger, first of all "Thank You", it was yours and Frances's book on LF and MF Photography that got me into 4x5.

    Here's what I understand, I'm not preaching to anyone:
    Anyways, back to the question at hand. As far as I understand it, an 18% gray card metered for a given environment (e.g. mid-afternoon, slightly overcast in September) and a negative exposed as per the meter reading will give a reasonable negative when developed as specified by the film manufacturer. This negative will then allow enough latitude for the lighter (Zone VI-VIII) and darker areas (Zone I-IV) to allow a reasonable print to be made.

    If you metered with an 18% gray card and your scene was snow, your negative will be exposed such that any non snowy white areas have been exposed for long enough to record detail on the negative.

    If you metered the snow and didn't add a stop or two over your meter's reading, then the negative will not have detail in the darker areas.

    So as far as my understanding goes, the meter is calibrated to expose an 18% gray on a standard film.

    My use of the Zone System is heavily based on this single point of reference. I point my spot at an area close to 18% gray if it's there and place this at Zone V. If I see a lighter area, maybe I put it at Zone VII or VIII. This is a more systematic approach to using an 18% gray then using your brain to add a stop or two.

    Alternatively, I meter for the darkest area I want detail in and then place this at Zone III.

    I might be way off base, but it seems to work for me ....

    Cheers !
    p.s. the scale on the Pentax Spotmeter really helps me make these decisions to my satisfaction. When I shot 35mm, I had no idea what my matrix-metering system was assuming ....
     

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